Regulating NO₂ adsorption at ambient temperature by manipulating copper species as binding sites in copper-modified SSZ-13 zeolites

Atmospheric NO₂ pollution poses significant risks to human health and the environment even at low concentrations, necessitating the development of efficient technologies for its removal under ambient conditions. In this study copper (Cu)-modified SSZ-13 zeolites (referred to as Cuⁿ⁺SSZ-13 where n represents the valence state of Cu) were developed for NO₂ removal by adsorption. Cuⁿ⁺SSZ-13 zeolites containing Cu species with different valence states and proportions were prepared by reducing a Cu²⁺-exchanged SSZ-13 zeolite (Cu²⁺SSZ-13) using H₂ at different temperatures. The Cuⁿ⁺SSZ-13 reduced at 190 °C showed the highest NO₂ removal capacity (1.79 mmol g⁻¹), outperforming pristine SSZ-13 and Cu²⁺SSZ-13 by 52.3% and 19.4%, respectively. The improvement was due to the increased amount of adsorption sites (Cu⁺ and H⁺) and the stronger affinity of Cu⁺ than Cu²⁺ for NO₂, as confirmed by density functional theory (DFT) calculations. The generation of Cu⁰ nanoparticles and moisture in zeolites during reduction was undesirable for NO₂ adsorption. However, this could be eliminated by lowering the reduction temperature and performing thermal activation, respectively. This work provides systematic methods for designing zeolite adsorbents for ambient NO₂ removal and offers insights into the burgeoning field of air pollution control.